Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) (Recurring, USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Total
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 416.8	$ 343.3
Total assets	417.1	344.3
Total liabilities	0	0.7
Total | Option Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0.3	1.0
Derivative liabilities		0.7
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	416.8	343.3
Total assets	416.8	343.3
Total liabilities	0	0
Level 1 | Option Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0	0
Derivative liabilities		0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0	0
Total assets	0	0
Total liabilities	0	0
Level 2 | Option Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0	0
Derivative liabilities		0
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0	0
Total assets	0.3	1.0
Total liabilities	0	0.7
Level 3 | Option Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0.3	1.0
Derivative liabilities		$ 0.7